A. Field of the Invention
The present invention relates to telecommunication devices. More particularly, the method of the present invention relates to a simplified modulation format. The method is readily applied to asymmetric digital subscriber line (ADSL) modems.
B. Description of the Related Art
Asymmetric Digital Subscriber Line
Asymmetric Digital Subscriber Line (ADSL) is a communication system that operates over existing twisted-pair telephone lines between a central office and a residential or business location. It is generally a point-to-point connection between two dedicated devices. ADSL supports bit transmission rates of up to approximately 6 Mbps in the downstream direction (to a subscriber device at the home), but only 640 Kbps in the upstream direction (to the service provider/central office). ADSL connections actually have three separate information channels: two data channels and a POTS channel. The first data channel is a high-speed downstream channel used to convey information to the subscriber. Its data rate is adaptable and ranges from 1.5 to 6.1 Mbps. The second data channel is a medium speed duplex channel providing bi-directional communication between the subscriber and the service provider/central office. Its rate is also adaptable and the rates range from 16 to 640 kbps. The third information channel is a POTS (Plain Old Telephone Service) channel. The POTS channel is typically not processed directly by the ADSL modemsxe2x80x94the POTS channel operates in the standard POTS frequency range and is processed by standard POTS devices after being split from the ADSL signal.
The American National Standards Institute (ANSI) Standard T1.413, the contents of which are incorporated herein by reference, specifies an ADSL standard that is widely followed in the telecommunications industry. The ADSL standard specifies a modulation technique known as Discrete Multi-Tone modulation.
Discrete Multi-Tone Modulation
Discrete Multi-Tone (DMT) uses a large number of subcarriers spaced close together. Each subcarrier is modulated during training using Quadrature Phase Shift Keying, or QPSK. During normal data transmission mode, the modulation used in ADSL is Quadrature Amplitude Modulation (MQAM). The data bits are mapped to a series of symbols in the I-Q complex plane, and each symbol is used to modulate the amplitude and phase of one of the multiple tones, or carriers.
In some ADSL transceivers, the symbols are used to specify the magnitude and phase of a subcarrier, where each subcarrier frequency corresponds to the center frequency of the xe2x80x9cbinxe2x80x9d associated with a Discrete Fourier Transform (DFT). The modulated time-domain signal corresponding to all of the subcarriers can then be generated in parallel by the use of well-known DFT algorithm called Inverse Discrete Fourier Transforms (IDFT). There are many well-known forms of the DFT and IDFT, often referred to generically as fast Fourier transforms (FFT) and inverse fast Fourier transforms (IFFT).
The symbol period in ADSL modems is relatively long compared to single carrier systems because the bandwidth available to each carrier is restricted. However, a large number of symbols is transmitted simultaneously, one on each subcarrier. The number of discrete signal points that may be distinguished on a single carrier is a function of the noise level. Thus, the signal set, or constellation, of each subcarrier is determined based on the noise level within the relevant subcarrier frequency band. The appropriate loading of each carrier is determined during initial training and analysis periods.
Because the symbol is relatively long and is preceded by a guard band, intersymbol interference is a less severe problem than with single carrier, high symbol rate systems. Furthermore, because each carrier has a narrow bandwidth, the channel impulse response is relatively flat across each sub-carrier frequency band. The DMT standard for ADSL, ANSI T1.413, specifies 256 sub-carriers, each with a 4.3125 kHz bandwidth. Each sub-carrier can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to 60 kbps per tone. DMT transmission allows modulation and coding techniques to be employed independently for each of the sub-channels.
The sub-channels overlap spectrally, but as a consequence of the orthogonality of the transform, if the distortion in the channel is mild relative to the bandwidth of a sub-channel, the data in each sub-channel can be demodulated with a small amount of interference from the other sub-channels. For high-speed wide-band applications, it is common to use a cyclic-prefix at the beginning to maintain orthogonality. Because of the periodic nature of the DFT, no discontinuity in the time-domain channel is generated between the symbol and the extension. It has been shown that if the channel impulse response is shorter than the length of the periodic extension, sub-channel isolation is achieved.
Time domain equalization is often performed to reduce the apparent length of the channel. impulse response. A frequency domain equalizer is able to remove channel distortion only in the case where the channel impulse response is shorter than the cyclic prefix. In full-rate ADSL data transmission, the receiver may perform time domain equalization prior to the conversion to the frequency domain in order to shorten the effective impulse response so that its length is less than the length of the cyclic prefix.
The signal processing associated with ADSL modems, specifically that associated with echo cancellation, equalization and DMT modulation/demodulation, is typically performed after the signal waveform is sampled. The processing is then performed on the digital signal. Because of the high bandwidth and the correspondingly high sample rate, the digital signal processing exceeds the capabilities of typical digital signal processors (DSP) that have been used quite successfully in the implementation of POTS-type modems. As a result, ADSL modems for subscriber devices, e.g., personal computers and other remote terminals, are significantly more expensive than traditional analog POTS-type modems. It has been generally understood that the increased cost is acceptable in view of the greatly enhanced data transmission capabilities of ADSL.
A sub-rate modulation method that is compatible with full-rate ADSL modems is provided. The method may be used to implement a reduced complexity transceiver, preferably remotely located from the central office or service provider. The method includes instructing the full-rate modem to transmit data using a subset of available carriers; receiving the subset of carriers at the remote location; and performing a reduced complexity time-to-frequency domain transform.
The full rate modem is instructed to use every other carrier, resulting in a transmitted time domain sequence that is cyclicxe2x80x94the first half of the symbol is identical to the second half. As a result, the first half of the symbol is treated as an extended cyclic prefix, resulting in the elimination of the need to perform time-domain equalization at the sub-rate ADSL modem receiver. This substantially reduces the amount of signal processing required by the sub-rate ADSL transceiver. In addition, local echo cancellation need not be performed during the first half of the received symbol period since these values are not utilized in the demodulation process.